Platform scales are used for high accuracy (legal for trade) measurement of loads. Such scales typically require an accuracy within 0.05 percent. Parallel beam load cells are frequently used in platform scales as the load sensing element. A typical parallel beam load cell will include two or three beams, with strain-sensitive portions. Strain-responsive elements, such as conventional strain gauges, are mounted on the strain-sensitive portions of the beams to provide, by means of an electrical bridge circuit and follow-on processing circuitry, a readout of the weight of the load on the platform.
One important criteria for such scales is that the indicated weight must be the same, within the required tolerance, regardless of the position of the load on the platform. This is often difficult to achieve.
Various techniques have been used to reduce the sensitivity of the load cell to the position of the load on the platform, otherwise known as moment balancing. The compensating techniques can take the form of electrical compensation and/or structural compensation. U.S. Pat. No. 4,128,001 to Marks is one example cf structural compensation. This compensation technique or some variation thereof, is widely used in the art. Marks teaches the removal, typically by filing, of material from the load cell beam, thereby changing the cross-sectional configuration of the beam, to accomplish moment balancing.
The Marks, technique, however, does have disadvantages. A primary disadvantage is that changing the cross-sectional configuration of the beam requires a new zero setting. Thus, the load must be removed, re-zeroing accomplished, and the load then returned after each attempt to moment balance the load cell. In addition, the moment balancing operation must be accomplished prior to the actual completion of the load cell, i.e., before sealing of the load cell is accomplished. Sealing of the load cell, however, can alter the response characteristics of the load cell, thus adding an immediate error to the just-balanced cell. Moment balancing of a parallel beam load cell is thus a time-consuming, and hence expensive, operation.
It is desirable that moment balancing be accomplished following the completion of manufacture of the load cell and in such a manner that the zero position is not significantly affected.